The significance of measurement of the hydrostatic intra-ocular pressure (IOP) is well known in medicine (ophthalmology). Excessive internal eye pressure is a cause of glaucoma and other eye diseases.
Professional medical staff, using a variety of methods—mainly the indentation tonometry, the applanation tonometry and the non-contact tonometry, perform common measurements of IOP. Several systems are available in the market. An indentation tonometry system is the Schiotz tonometer, which dates back to the nineteen thirties. A more recently developed system is the Mentor Tono-Pen XL from Mentor O&O, Inc. of Norwell, Ma US. An applanation tonometry system is the Goldman tonometer, which is a standard tonometer in a considerable amount of medical institutions. The above mentioned systems and methods are based on direct contact with the corneal eye associated with liquids and medications applied to the eye. A non-contact tonometry is preferable since it decreases hazards to the tested eye associated with the measurement process. A non-contact tonometry system is the puff tonometer XPERT NCT from Reichert Cambridge US. Puff tonometry employs detecting changes in the reflectance of a cornea, which is distorted by a pneumatic pulse. The measurement process basically includes the steps of: (i) placing and aligning the tonometer in front of the tested eye; (ii) measuring the intensity of a light reflected by a test area within the surface of the cornea; (iii) distorting the cornea by projecting a pneumatic pulse towards it, and (iv) matching features of the intensity-time profile of the light reflected by the distorted cornea with IOP values. The accuracy of IOP values obtained is considerably affected by misplacement or misalignment of the tonometer. Therefore allowed tolerances of placement and alignment are significantly narrow.
Once a patient is diagnosed as suffering from excessive eye pressure, he/she is to have IOP monitored periodically—typically at different hours during the day. It is desirable that a user is be able to measure his or her own IOP at home, rather than make special visit to the clinic for this purpose. However, current commercially available devices providing for self—examination of IOP are not satisfactory mainly due to accuracy and cost considerations.
Therefore, efforts to develop a self-operated tonometer suitable for home medicine are ongoing. U.S. Pat. No. 6,440,070 discloses a device consisting of a gauge, which is pushed against the eyelid while force is measured. An ultrasound-measuring device is also used to measure a distance from an internal object within the tested eye. Correlating the force applied to the displacement of the pressed surface caused by this applied force provides assessment of pressure. U.S. Pat. No. 6,746,400 discloses a system implementing of a plurality of pressure sensors providing also a spatial distribution of pressure from which IOP can be derived. Both above mentioned inventions involve direct or indirect contact with the tested eye. Measured IOP values obtained thus are considerably less sensitive to fitting or aligning of the measuring gear as compared to those obtained by current systems employing optical measurements. However, patients tend to decline using a measuring system which entails applying force by touching the eye.